The invention relates to a detection assembly designed to measure the neutron fluence rate outside the core of a nuclear reactor. The measuring range is very extensive and neutron detection is ensured under normal conditions and under exceptional conditions characterized by a very high gamma dose rate. According to the former art, several detectors had to be used to cover the whole measuring range of the fluence rate levels of a light water reactor. For low levels, proportional counters are used delivering pulses whose count enables the neutron fluence rate to be ascertained. For high levels, ionization chambers are used delivering a current whose measured value is proportional to the fluence rate sought for.
Two types of ionization chambers are used:
a non-compensated ionization chamber to make an accurate measurement at high level; and
an ionization chamber compensated for gamma radiation to ensure continuity of detection between the low level and the high level.
The fission chamber is an ionization chamber filled with gas which enables mainly the thermal neutrons to be detected by reaction on a lining of fissile material, generally formed by uranium 235.
The fission fragments released by the reaction with the thermal neutrons ionize the filling gas and this results in pulses whose count enables the neutron fluence rate to be ascertained. Like all detectors using ionization phenomena in gases, a fission chamber is sensitive to gamma radiation, whose contribution is to be eliminated.
The amplitude of the pulses delivered by the chamber following the reactions of the neutrons with the uranium 235 is much greater than the amplitude of the pulses induced by the reactions of the gamma rays with the detector materials. Using a conventional amplitude discrimination technique, it is possible to count only the pulses caused by the neutrons and to totally eliminate the pulses caused by the gamma rays.
Given that the amplitude of the neutron pulses is much greater than that of the gamma pulses, the fission chamber enables the neutrons to be detected in very high gamma dose rates corresponding to the exceptional conditions envisaged.
The range of use of fission chambers is characteristically defined by the count rate limits which are closely linked to the pulse width. The maximum count rate is limited to 10.sup.6 counts per second. The minimum count rate is imposed by the statistical limits and corresponds to one count per second. A particularity of fission chambers lies in the wide variety of sensitivities accessible by adjusting the quantity of fissile material introduced into the detector. Fission chamber sensitivities are generally comprised between 4 counts per second per flux unit and 10.sup.-3 counts per second per flux unit. The flux unit (nv) is equal to one neutron per square centimeter and per second.
The boron-lined compensated ionization chamber is a differential chamber formed by three electrodes which define two elementary chambers. The first elementary chamber is located between the positive high voltage polarization electrode and the signal electrode which collects the charges developed by ionization in the gas. The surfaces of the electrodes of this elementary chamber are covered with a boron lining. This elementary chamber is therefore sensitive to neutrons and to gamma rays.
The second elementary chamber is located between the signal electrode and the compensation electrode which receives a negative polarization voltage. As this elementary chamber has no boron lining, it is sensitive to gamma rays only. The ionization currents collected on the signal electrode are eliminated due to their opposing polarizations. The resulting current is mainly the image of the neutron fluence rate sought for.
The upper neutron fluence rate detection limit is imposed by the saturation characteristics, which depend on the surface quantity of sensitive material, the voltage applied and the filling gas.
Adjustment of these parameters enables the upper detection limit, which may reach 10.sup.11 flux units, to be determined. The lower detection limit is imposed by the compensation ratio, the neutron fluence rate and the gamma dose rate.
Under normal conditions, in a pressurized water reactor, the gamma dose rate liable to disturb measurement is about 10 Gy/h. The minimum measurable neutron fluence rate is about 10.sup.4 NV units.
Under exceptional conditions, it is admitted that the gamma dose rate can reach 10.sup.4 Gy/h and the compensation ratio can be modified. The range of use is therefore reduced. The lower detection limit is typically about 10.sup.8 NV.
The use of several types of chambers or counters with independent housings to cover the whole fluence rate measuring range of a light water nuclear reactor gives rise to a problem of dimensions and requires a multitude of connecting cables.
It is preferable to be able to use a detection assembly to:
limit the number of cables;
operate under exceptional conditions; and
increase the measuring range.